Printing devices, including electrophotographic printing devices, require a system and method for achieving proper IOP registration. In a xerographic printing device, IOP registration may be achieved by controlling registration of an image bearing surface, such as a photoreceptor belt, an intermediate transfer belt if any, images to be transferred, and the substrate to which the image will be transferred.
IOP misregistration may be determined by measuring image offsets in the process and cross-process directions, image magnification in the process and cross-process directions, paper skew, and image skew. The process direction is the direction in which the substrate onto which the image is transferred and developed moves through the image transfer and developing apparatus. The cross-process direction, along the same plane as the substrate, is substantially perpendicular to the process direction. Paper skew is the angular deviation from the process direction of the substrate as it travels past the transfer zone. Image skew is the angular deviation of the raster output scanner scan lines from the process direction of the substrate, or a line normal to the process direction of the marked substrate.
Measurements such as those listed above may be made by printing a diagnostic image and taking measurements of the printed image. The printed image may be measured by hand using a magnifying eye loupe or may be scanned in and performed automatically. The results are then provided, typically manually, to a control system of the printing device. The control system uses the measurements to make adjustments for correcting any detected misregistration. The above process is performed offline (not inline), and requires human intervention, with the potential for human error. Moreover, this process is extremely time consuming, e.g., approximately forty-five minutes per page, and to obtain increased precision and accuracy and to minimize the effects of variability in the printing device, multiple diagnostic images may be printed, e.g., at least three images or as many as ten or more images. Unfortunately, increasing the number of diagnostic images does not reduce the time necessary for calibrating a printing device, and thus, each diagnostic image analyzed incurs approximately forty-five minutes of measurement time. As these measurements often occur during the assembly or installation of a printing device, the length of time for performing the assembly or installation are increased, and for the higher quality printing, such measurements may be made once per day or prior to beginning each print job.
In view of the foregoing, it should be appreciated that printing images, especially high quality duplex printing of images, i.e., double-sided printing, is difficult to perform while maintaining document-to-document and/or side-to-side accuracy. Some printing tolerances require image placement as accurate as 0.8 millimeters (mm) on a single sided document, which results in an accuracy requirement of 1.6 mm on a two sided document. Newer printing requirements further reduce printing tolerances to 0.2 mm for single sided documents and 0.4 mm for double sided documents. Under ideal conditions, the best result obtainable by manual measurement is about 0.25 mm to 0.3 mm for single sided documents; however, as described above, such measurements are extremely time consuming. Moreover, additional variability is introduced when different people perform the same measurements.
Meeting such stringent printing requirements is difficult, and a variety of solutions have been developed for attempting to accomplish such tolerances. Several solutions are described in the U.S. patents and patent applications included above; however, each of these solutions has drawbacks and deficiencies. For example, some of these solutions are limited to measuring skew on a leading edge of a document only, or merely attempt to quantify the above described measurements after an image has been printed, e.g., post fusing of toner on paper. Such post fusing measurements carry inherent errors merely by being taken after the fusing step. For example, the paper changes size as it passes through the fuser assembly, i.e., the paper shrinks due to evaporation of water or is stretched due to pressure on the paper. Moreover, depending on the type of paper, the extent of changes in size may vary, e.g., thinner paper changes more, e.g., 0.1-0.4% changes in size. Thus, for larger sizes of paper such as twenty-two inch paper, errors may be as great as 3-4 mm across the full length.
Furthermore, although image placement on an image bearing surface such as a photoreceptor belt may be tightly controlled, e.g., within thirty-five microns, variability in paper movement through the paper path introduces a wide variability in IOP registration. For example, speed control can make absolute measurements difficult to obtain. The leading edge of a piece of paper may be accurately determined; however, variability in the paper speed will affect image placement, and as such errors get integrated over longer distances, the errors accumulate, further increasing the difficulty of obtaining accurate absolute measurements.
The present disclosure addresses a system and method for automatically calibrating IOP registration of printed images in single and double sided printed documents and further calibrating IOP registration of printed images across multiple discrete documents.